The preferred method and embodiment for practicing the present invention is particularly directed to an ink jet printer wherein a print head scans over a print medium, most typically a sheet of paper or transparent film, by shuttling back and forth across the sheet (bi-directional movement) or by moving continuously along the sheet in one direction while the sheet is held against a rotating drum. Images are formed by selectively and serially depositing ink drops of primary or base colors at uniformly spaced address locations disposed in uniformly spaced rows to form a dot-matrix image. Variations in color may be achieved by depositing one or more ink drops of more than one size or color at an address to form picture elements or pixels.
The present invention however is equally applicable to any printing process wherein a print head travels along parallel lines relative to a print medium to form a desired final image, whether the image be graphic or textual. In the following text, the term "print" is considered to include the general situation where a print element or nozzle addresses an ink drop location, whether or not ink is deposited. In the general situation the size of the drop may vary and even the number of drops of a given color that are deposited at a particular address can vary. Hewlett-Packard Labs has demonstrated the latter with drop-on-demand (DOD) thermal ink jets; and Hertz, at the Lund Institute in Sweden, has also demonstrated this with continuous ink jets. Printing with drops of several selected sizes (for gray scale control at each address) was demonstrated by MRIT with air assisted DOD jets in the early 1980s.
Print heads are known that contain a nozzle for each color of printing for a single line. These nozzles are positioned adjacent to a sheet of paper. A print head carriage then moves relative to the paper one line at a time depositing ink pixels at selected pixel locations until the entire image area has been scanned.
Representative of the prior art techniques is that disclosed in U.S. Pat. No. 4,630,076 issued to Yoshimura for "Ink-On-Demand Color Ink Jet System Printer". The devices disclosed therein show a plurality of sets of jet or nozzle arrays providing printing of all of the colors on each of a given set of print lines in a single scan of the print head (band printing). These devices print the color drops in one order when the print head is travelling in one direction, and in the reverse order when travelling in the other direction. This printer thus does not provide any form of interlacing: band, line, or color. Inks that bleed when printed therefore will mix within colors on a single line as well as between lines.
A variation of this technique is illustrated in U.S. Pat. No. 4,593,295 issued to Matsufuji et al. for "Ink Jet Image Recording Device with Pitch-Shifted Recording Elements". A double set of printing arrays are disclosed and offset in the direction of relative print medium movement so that the colors can be printed in the same order for both scan directions. As with the printer of Yoshimura, this printer prints all of the colors on a single line in a single pass of the print elements over a set of print lines.
Other ink jets have more than one nozzle to print a given color on each address of a given line. One nozzle is used to print ink at its maximum optical density, and the other(s) to print ink at some diluted dye concentration(s) so that more than one optical density level of the color can be obtained at each address. Again, such techniques involve the near simultaneous depositing of ink drops on pixel or image elements that are effectively in adjacent lines or in the media advance direction, as well as on the same pixel or image element. The resulting bleeding produces visually perceptible lines in the direction of print head traverse or scan across a print medium.
Some early printers also had the nozzles aligned normal to the scan direction for scanning spaced-apart parallel lines. Thus, colors are always laid down in the same sequence, and one color has time to dry before the next one is printed on top of it. Such systems do not provide for color, line or band interlacing, since printing is done with a single nozzle for each color.
Hirata et al., in U.S. Pat. No. 4,554,556 entitled "Color Plotter", disclose printing a dot with all three colors at once, or sequentially during a single scan. Tozaki, in U.S. Pat. No. 4,580,150 entitled "Recording Apparatus", disclosed a print array in which two nozzles are used to print one color in a limited image region and then a single nozzle is used to print a second color over the same region. These systems produce bands of print, print multiple colors in a single scan, and do not provide interlacing.
An example of band color printing in which the color arrays are spaced in the scan direction is disclosed by Helinski et al. in U.S. Pat. No. 4,714,936 entitled "Ink Jet Printer". A black array is also provided that has more nozzles than those in the individual color arrays. No band, line or color interlacing is provided. All colors are deposited on a line in a single scan, so mixing of inks occurs.
A form of line interlacing of band color printing is disclosed by Hillmann et al. in U.S. Pat. No. 4,728,968 entitled "Arrangement of Discharge Openings in a Printhead of a Multi-Color Ink Printer". For letter quality printing, the array is moved one half the draft-quality line spacing to print higher resolution images. This requires a different print medium advance after alternate scans. Again, all of the colors in a given line are printed during a single scan of the print head across the medium.
Color arrays spaced in the direction of print medium movement are also disclosed in the references. Logan, in U.S. Pat. No. 4,680,596 entitled "Method and Apparatus for Controlling Ink-Jet Color Printing Heads", discloses such arrays for printing dots in pixels to vary color tone. In this patent, three dot rows, forming a single pixel row, are printed with each color during each scan. This, then, is a form of solid band printing of each color. The head measures about two inches by three inches. There is no band or line interlacing of colors. Further, with multiple ink drops per pixel per scan, there is mixing of ink of the same color, which creates line artifacts.
Another example of color-band-printing arrays spaced in the direction of medium movement is disclosed by Chan et al. in U.S. Pat. No. 4,812,859 entitled "Multi-Chamber Ink Jet Recording Head for Color Use". Four heads, one for each primary color and black, print adjacent solid bands. Band artifacts are thus produced and there is no line, band or color interlacing.
In band printing by color arrays spaced in the direction of print medium movement, each color dries before the next color is deposited, and the colors are always deposited in the same sequence. When the color arrays are spaced only in the direction of scan movement, all the colors are deposited during each scan and the sequence of deposition is reversed for the two scan directions.
Prints generated by some serial dot-matrix color printers exhibit noticeable streaks parallel to the pen scan direction in areas printed in solid colors. These streaks can be either higher or lower in optical density than the surrounding area and occur where a band of color printed in one scan abuts a band of color printed in the next scan. Mechanical errors in paper advance mechanisms and ink bleeding are two of the causes for this. To minimize the effect, the bands of color should be interlaced rather than abutted. As discussed herein, band interlacing of a color refers to the partial overlapping of a first printed band of the color with a subsequent printed band of the same color. This also requires line interlacing and results in the spacing apart of any printing defects due, for example, to a defect in a single printing element.
Line interlacing means that adjacent lines of dots of the same color are printed in sequential scans of the pen. For example, lines 1, 3, 5, etc., might be printed in one scan, while lines 2, 4, 6, etc., would be printed in the next scan. In a high speed printer, it is desirable to print in both scan directions. With line interlacing, any printing errors and hence image defects that might be dependent on the scan direction would be generated at the spatial frequency of the inverse line spacing and should be less noticeable than if they were generated at a lower spatial frequency.
Different types of inks are used in drop-on-demand printing. These are primarily water-based inks, oil-based inks, and hot-melt or thermoplastic inks. The latter inks are preferred, due to the intensity of the colors and the fact that they can be used on many different print mediums. A discussion of printing with colored inks, generally, and with hot-melt inks, in particular, is discussed by Howard et al. in U.S. Pat. No. 4,741,930 entitled "Ink Jet Color Printing Method". This patent specifically discloses the ink itself, rather than a printing process, other than disclosing that it is desirable to apply the different colors of ink to a spot after the prior application has set.
If dots of hot-melt ink that have not set are deposited continuously together or on top of each other, they mix. When they mix, the resultant color is different than it is if the first dot solidifies before the second dot is deposited. The color laydown sequence is also important. Different sequences produce color hue shifts and appearances of surface irregularities.
Ideally then, each of the multicolor overlay sequences should always be the same regardless of scan direction. If this is not possible, then the next best thing is to have the sequences alternate on adjacent lines so that the spatial frequency of the hue variations will be as high as possible and will be averaged out as much as possible by the visual system of an observer.
It can therefore be seen that it is desirable to provide line interlacing of each of the colors, band interlacing of each of the colors, and constant overlay sequence for each of the two-color combinations when printing bi-directionally.